A digital processing system typically contains a memory device for data storage. The memory device, for example, is generally categorized volatile memory and/or nonvolatile memory device. Volatile memory includes random-access memory (“RAM”), static random-access memory (“SRAM”), and/or dynamic random-access memory. Nonvolatile memory (“NVM”) can be NAND flash memory, NOR flash memory, phase-changing memory, Electrically Erasable Programmable Read-Only Memory (“EEPROM”), and Silicon-Oxide-Nitride-Oxide-Silicon (“SONOS”) memory, and the like.
NVM, such as NAND or NOR based flash memory, is widely used in today's computing world, and its unique cell structure provides a small, high density, and low write current storage device. NVM such as NAND based flash memory has become a major persistent storage memory for various devices and systems, such as memory cards, USB flash drives, and solid-state drives. Some exemplary applications of flash memory include personal computers, PDAs, digital audio players, digital cameras, mobile phones, synthesizers, video games, scientific instrumentation, industrial robotics, and medical electronics. With technologic improvement, the NVM technology such as NAND flash memory has reached 16 nanometers (“nm”), and its single-chip density can reach 128 gigabit (“Gb”) storage capacity. A drawback associated with NAND flash memory, however, is slow read and/or write operation. For example, a typical read operation may take 25 nanoseconds (“ns”) and an erase operation can take two (2) milliseconds (“ms”). A page write operation can take up to 300 μs to complete. The slow operation can negatively impact the overall system performance.
DRAM typically provides relatively fast read and/or write operation. However, DRAM is a volatile memory wherein the stored data may disappear when the power is down. Also, DRAM cell size is typically larger than NAND cell size whereby it generally has higher cost and lower density. For a typical computing system, NVM such as NAND based flash memory is used for main storage while DRAM is used for working memory. For example, data stored in NVM can be loaded into DRAM for processing and executing, and the result may be subsequently written back to NVM when the task is done.
Since the density of DRAM is generally lower than the density of NVM such as NAND flash memory, NAND and DRAM are generally fabricated in separate chips or dies due to different processes. However, two-chip solution not only increases the chip count, footprint, system cost, but also slows down the system performance due to data transfer between the two chips.